1. Field of the Invention
The present invention relates generally to a misfire detecting method for deciding or detecting occurrence of misfire on the basis of variation in rotation speed (rpm) of an internal combustion engine and an apparatus for carrying out the method. More specifically, the invention is concerned with misfire detecting method and apparatus which are so arranged that erroneous misfire decision due to variation in the rotation speed of the engine as brought about by resonance phenomenon taking place in a driving system can be excluded.
2. Description of Related Art
Heretofore, the misfire detecting apparatus which is so arranged as to make decision concerning occurrence of misfire on the basis of variation in the rotation number or speed (rpm) of the internal combustion engine is well known in the art, as is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 194346/1992 (JP-A-4-194346). More specifically, in the conventional misfire detecting apparatus known heretofore, a pulse period is measured or determined for every unit crank angle on the basis of a crank angle signal, whereon a mean angular velocity for every unit crank angle is differentiated three times to thereby determine a value which is used as an index indicating variation (also referred to as the variation index) of the pulse period and hence that of rotation speed of the internal combustion engine. Decision as to occurrence of misfire is realized by comparing the variation index as determined with a predetermined decision reference value.
For having better understanding of the present invention, the background techniques thereof will be described in some detail. FIG. 6 is a schematic diagram showing the conventional apparatus having a misfire detecting function as disclosed in the above publication.
Referring to the figure, mounted on a crank shaft 2 which is rotationally driven by an internal combustion engine (hereinafter referred to simply as the engine) 1 is a sensing blade member 3 having segment-like blades disposed fixedly the rotational direction so as to rotate together with the crank shaft 2.
On the other hand, a crank angle sensor 4 is stationarily mounted on the engine body 1, facing in opposition to the segment blade member 3, for detecting edges of the segment-like blades to thereby output a crank angle signal .theta. containing a sequence of pulses each generated for each unit crank angle.
Each of the engine cylinders 5 which defines combustion chambers of the engine 1 is surrounded by a space through which cooling water 6 is forced to flow. For detecting the temperature Tw of the cooling water 6, there is provided a water temperature sensor 7. Further, each of the engine cylinders 5 is provided with an intake valve 8, an exhaust valve 9 and an ignition plug 10 which are driven at predetermined cycles or timings, respectively.
Although only one engine cylinder 5 is shown in FIG. 6, this is only for simplification of illustration. It goes without saying that a plurality of cylinders (e.g. four or six cylinders) may be incorporated in the engine in juxtaposition to one another. Further, the crank angle signal .theta. is commonly referred to as the reference position signal SGT, wherein the falling edge of each pulse contained in the crank angle signal .theta. corresponds to the reference position for each of the individual engine cylinders.
The intake valve 8 and the exhaust valve 9 are opened and closed in accordance with a control procedure of the engine 1 in such a manner that the engine cylinder 5 is communicated through the intake valve 8 to an intake pipe 11 during an intake (or suction) stroke, while it is communicated through the exhaust valve 9 to an exhaust pipe 12 during an exhaust stroke. On the other hand, the ignition plug 10 is applied with a high voltage for generating a spark at a proper time point in a combustion/expansion stroke which succeeds to a compression stroke, whereby air-fuel mixture gas charged in the engine cylinder 5 is ignited.
Provided in association with the intake pipe 11 are a fuel injector 13 for injecting fuel upstream of the intake valve 8 to thereby produce an air-fuel mixture, an air flow sensor 14 for detecting an intake air flow Qa at an air intake port, a throttle valve 15 disposed at a position downstream of the air flow sensor 14 and driven to be opened or closed, a throttle position sensor 16 for detecting an opening degree .alpha. of the throttle valve 15 and a pressure sensor 17 for detecting the intake pipe pressure Pa prevailing within the intake pipe 11.
The detection information signals .theta., Tw, Qa, .alpha. and Pa outputted from the above-mentioned sensors 4, 7, 14, 16 and 17, respectively, are inputted to an electronic control unit (ECU for short) 20 which may be constituted by a microcomputer.
Of the detection information signals mentioned above, at least one of the intake air flow Qa outputted from the air flow sensor 14, the throttle opening degree .alpha. derived from the output of the throttle position sensor 16 and the intake pipe pressure Pa detected by the pressure sensor 17 is made use of as load information which indicates a load state of the engine 1.
The ECU 20 serving as the conventional engine control apparatus incorporating the misfire detecting function is comprised of an input/output interface, an analogue-to-digital or A/D converter, a time counter, a read-only memory or ROM, a random access memory or RAM and so forth (not shown) for arithmetically determining control parameters on the basis of the various engines operations state indicating signals .theta., Tw, Qa, .alpha. and Pa as detected, to thereby generate an ignition signal G for the ignition plug 10 and a fuel injection signal J for the injector 13.
The ECU 20 is so arranged as to make decision concerning occurrence of misfire on the basis of the crank angle signal .theta. to thereby output a misfire decision signal H when occurrence of misfire event is detected. In that case, a display device 30 is activated to generate a message of occurrence of misfire to the operator.
FIG. 7 is a block diagram for illustrating functions of a conventional misfire detecting apparatus known heretofore which is incorporated in the ECU 20 shown in FIG. 6.
Referring to FIG. 7, a sensor block 18 represents correctively the crank angle sensor 4, the water temperature sensor 7, the air flow sensor 14, the throttle position sensor 16 and the pressure sensor 17, and thus operation information D outputted from the sensor block 18 includes the crank angle signal .theta., the temperature Tw, the intake air flow Qa, the throttle opening degree .alpha., and the intake pipe pressure Pa mentioned above, while an engine control signal E outputted from the ECU 20 contains the ignition signal G and the fuel injection signal J mentioned previously.
The ECU 20 in turn is composed of a period measuring means 21 for measuring or determining a pulse period or duration T.theta. contained in the crank angle signal .theta. for every unit crank angle, a variation index arithmetic means 22 for arithmetically determining the variation index (mentioned hereinbefore and designated by K) indicating variation of the pulse period T.theta. and hence variation in the rotation number or speed (rpm) of the engine 1 and a misfire decision means 23 for making decision concerning occurrence of misfire on the basis of the variation index K to thereby output a misfire decision signal H upon decision of occurrence of the misfire event.
The crank angle signal .theta. represents the rotation number or speed (rpm) of the engine 1. Thus, the period measuring means 21 constituting a part of the ECU 20 also serves as an engine rotation speed (rpm) detecting means.
On the other hand, the period measuring means 21 and the variation index arithmetic means 22 cooperate to constitute a rotation speed variation detecting means for detecting variation in the rotation speed (rpm) of the engine 1, while the misfire decision means 23 cooperate with the period measuring means 21 and the variation index arithmetic means 22 to constitute a major part of the misfire detecting apparatus.
Furthermore, the ECU 20 includes an operation state decision means 24 for determining or deciding the operation state of the engine 1 on the basis of the crank angle signal .theta. and the operation information D, and an engine control means 25 for generating an engine control signal E on the basis of the operation state of the engine as detected and the misfire decision signal H.
Next, referring to a waveform diagram shown in FIG. 8, description will be directed to operation of the conventional misfire detecting apparatus shown in FIGS. 6 and 7.
In FIG. 8, an angular velocity .omega. is equivalent to the rotation speed (rpm) of the engine 1 and a tertiary variation of a mean angular velocity V (i.e., a value obtained by differentiating three times the mean angular velocity V) is equivalent to the variation index K of the pulse period or duration T.theta..
Parenthetically, although the value obtained by differentiating the mean angular velocity V three times (hereinafter referred to as the tertiary variation) is used as the variation index K for the misfire decision, a primary variation .DELTA.V of the mean angular velocity V (i.e., mean angular acceleration obtained by differentiating once the mean angular velocity) or a secondary variation .DELTA.W (i.e., value obtained by differentiating twice the mean angular velocity V) may be used as the variation index K.
Furthermore, in the case of the example illustrated in FIG. 8, it is assumed that the variation index K is compared with decision values Ck (where k=i-2, i-1 and i, respectively) which corresponds to a pre-preceding index value K.sub.i-2, a preceding index value K.sub.i-1 and a current index value K.sub.i, respectively.
In the normal operation state of the internal combustion engine, the engine control means 25 incorporated in the ECU 20 generates the ignition signal G and the fuel injection signal J conforming to the engine operation state determined on the basis of the crank angle signal .theta. and the operation information D for thereby controlling the operation of the engine 1.
On the other hand, the period measuring means 21 measures or determines the pulse period or duration T.theta. between the successive falling edges of the pulses contained in the crank angle signal .theta., while the variation index arithmetic means 22 arithmetically determines the angular velocity .omega. on the basis of the pulse period or duration T.theta. (see FIG. 8).
In addition, the variation index arithmetic means 22 determines the mean angular velocity V for the angular velocity .omega. and then determines on the basis of the mean angular velocity V a difference (V.sub.i -V.sub.i-1) between the preceding value V.sub.i-1 and the current value V.sub.i of the mean angular velocity V as the primary variation .DELTA.V, a difference (.DELTA.V.sub.i -.DELTA.V.sub.i-1) between the preceding value .DELTA.V.sub.i-1 of the primary variation .DELTA.V and the current value .DELTA.V.sub.i as the secondary variation .DELTA.W, while determining a difference (.DELTA.W.sub.i -.DELTA.W.sub.i-1) between the preceding value .DELTA.W.sub.i-1 of the secondary variation .DELTA.W and the current value .DELTA.W.sub.i as the tertiary variation, i.e., the variation index K.
The misfire decision means 23 compares the pre-preceding value K.sub.i-2, the preceding value K.sub.i-1 and the current value K.sub.i of the variation index K with the pre-preceding decision reference value C.sub.i-2, the preceding decision reference value C.sub.i-1 and the current decision reference value C.sub.i, respectively, wherein when the conditions mentioned below are concurrently satisfied, the misfire decision means 23 decides that the misfire event occurs at a higher probability than a predetermined value, to thereby issue a misfire decision signal H. EQU K.sub.i-2 &lt;C.sub.i-2 EQU K.sub.i-1 &gt;C.sub.i-1 EQU K.sub.i &gt;C.sub.i
In FIG. 8, there is illustrated such situation in which the state meeting the misfire conditions mentioned above takes place only once between the normal combustion strokes.
Under the circumstances, the misfire decision signal H is generated. The engine control means 25 then responds to the misfire decision signal H to thereby disable or inhibit application or supply of the fuel injection signal J to the fuel injector 18 of the engine cylinders 5 for which decision is made that the misfire occurred, to thereby prevent exhaust of the gas mixture undergone insufficient combustion.
On the other hand, the display device 30 is driven in response to the misfire decision signal H to massage occurrence of the misfire event to the operator.
In this conjunction, it is however noted that in certain operation states such as low-speed or low-load operation state, single-shot misfire which can or should be neglected is likely to occur. Besides, resonance with the driving system such as transmission which is operatively coupled to the crank shaft of the engine may often take place due to external disturbances or the like. Such being the circumstances, the variation index K may meet the misfire determination conditions mentioned previously, notwithstanding of the fact that the combustion stroke of the engine is normal.
For the reasons mentioned above, misfire decision may erroneously be made even in the normal operation state of the engine, whereby the fuel injection is unnecessarily cut off with the misfire indication being generated, in operation states such as low-speed operation state of the engine, low-load operation state thereof or the like, giving rise to problem.
As will now be apparent from the above description, the conventional misfire detecting apparatus suffers such problem that misfire decision may erroneously be made in the low-speed or low-load operation state of the engine, because decision as to occurrence of the misfire is made only on the basis of the variation index K.